setText和OnClickListener不起作用

Question

我是Android新手，所以我面臨很多問題。我要做一個簡單的程序，它可以從edittext中獲取一個值，並通過單擊按鈕在textview中打印它。這裏是我的代碼：

程序說，不幸的是它停止工作。但是，當我從setOnClickListener刪除最後幾行時，它工作正常。我也只通過setText測試了它。它也不起作用。謝謝。 logcat的：

XML：

Answer 1

你的代碼應該是PlaceHolderFragment，像下面..

public class PlaceHolderFragment extends Fragment { 
    private EditText et; 
    private Button b; 
    private TextView tv; 

    @Override 
    public View onCreateView(LayoutInflater inflater, ViewGroup container, 
     Bundle savedInstanceState) { 
     View rootView = inflater.inflate(R.layout.fragment_main, container, 
      false); 
     return rootView; 
    } 

    @Override 
    public void onActivityCreated(@Nullable Bundle savedInstanceState) { 
     // TODO Auto-generated method stub 
     super.onActivityCreated(savedInstanceState); 

     et=(EditText) getView().findViewById(R.id.EditText1); 
     b = (Button) getView().findViewById(R.id.Button1); 
     tv = (Button) getView().findViewById(R.id.textView3); 
     b.setOnClicklistener(new OnClickListener(){ 
      @Override 
      public void onClick(View arg0){ 
       tv.setText("Pressed"); 
      } 
     }); 
    } 
} 

而且從onCreate()

et=(EditText) findViewById(R.id.EditText1); 
b = (Button) findViewById(R.id.Button1); 
tv = (Button) findViewById(R.id.textView3); 
b.setOnClicklistener(new OnClickListener(){ 
    @Override 
    public void onClick(View arg0){ 
     tv.setText("Pressed"); 
    } 
}); 

刪除這些代碼試試這個...

更新

1. activity_main.xml中

fragment_main.xml

MainActivity.java

 
import android.support.annotation.Nullable; 
import android.support.v7.app.ActionBarActivity; 
import android.support.v4.app.Fragment; 
import android.os.Bundle; 
import android.view.LayoutInflater; 
import android.view.Menu; 
import android.view.MenuItem; 
import android.view.View; 
import android.view.View.OnClickListener; 
import android.view.ViewGroup; 
import android.widget.Button; 
import android.widget.EditText; 
import android.widget.TextView; 

public class MainActivity extends ActionBarActivity { 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 
     if (savedInstanceState == null) { 
      getSupportFragmentManager().beginTransaction() 
        .add(R.id.container, new PlaceholderFragment()).commit(); 
     } 
    } 

    @Override 
    public boolean onCreateOptionsMenu(Menu menu) { 
     // Inflate the menu; this adds items to the action bar if it is present. 
     getMenuInflater().inflate(R.menu.main, menu); 
     return true; 
    } 

    @Override 
    public boolean onOptionsItemSelected(MenuItem item) { 
     // Handle action bar item clicks here. The action bar will 
     // automatically handle clicks on the Home/Up button, so long 
     // as you specify a parent activity in AndroidManifest.xml. 
     int id = item.getItemId(); 
     if (id == R.id.action_settings) { 
      return true; 
     } 
     return super.onOptionsItemSelected(item); 
    } 

    /** 
    * A placeholder fragment containing a simple view. 
    */ 
    public static class PlaceholderFragment extends Fragment { 

     private EditText et; 
     private Button b; 
     private TextView tv; 

     public PlaceholderFragment() { 
     } 

     @Override 
     public View onCreateView(LayoutInflater inflater, ViewGroup container, 
       Bundle savedInstanceState) { 
      View rootView = inflater.inflate(R.layout.fragment_main, container, 
        false); 
      return rootView; 
     } 

     @Override 
     public void onActivityCreated(@Nullable Bundle savedInstanceState) { 
      // TODO Auto-generated method stub 
      super.onActivityCreated(savedInstanceState); 

      et = (EditText) getView().findViewById(R.id.editText1); 
      b = (Button) getView().findViewById(R.id.button1); 
      tv = (TextView) getView().findViewById(R.id.textView1); 
      b.setOnClickListener(new OnClickListener() { 
       @Override 
       public void onClick(View arg0) { 
        tv.setText("Pressed"); 
       } 
      }); 
     } 
    } 
} 

輸出：

Answer 2

我想你共享XML是fragment_main不activity_main。確保你正在膨脹正確的xml，並且只能參考它的控制... NULL Pointer Exception正在生成，因爲只有。

根據你的代碼，TextView的，和的EditText按鈕必須駐留在activity_main.xml只有

et=(EditText) findViewById(R.id.EditText1); 
b = (Button) findViewById(R.id.Button1); 
tv = (TextView) findViewById(R.id.textView3); 

確認。

如果控制在frgament_main，你需要使用這個代碼：

public class PlaceHolderFragment extends Fragment { 
    private EditText et; 
    private Button b; 
    private TextView tv; 

    @Override 
    public View onCreateView(LayoutInflater inflater, ViewGroup container, 
     Bundle savedInstanceState) { 
     View rootView = inflater.inflate(R.layout.fragment_main, container, 
      false); 

     et=(EditText) rootView.findViewById(R.id.EditText1); 
     b = (Button) rootView.findViewById(R.id.Button1); 
     tv = (TextView) rootView.findViewById(R.id.textView3); 
     b.setOnClicklistener(new OnClickListener(){ 
      @Override 
      public void onClick(View arg0){ 
       tv.setText("Pressed"); 
      } 
     }); 
     return rootView; 
    } 

}